barricades. not the type of holiday ornaments you would expect to see around the christmas tree. >> we have seen some of the cops carrying what look like either ak-47s or assault rifles. >> just a little extra alert. keep an extra eye on your kids and bags. >> reporter: this attack on a berlin marketplace left 12 people dead, a fresh reminder to major cities across the world on holiday alert. in london, more police patrolling the palace. >> the policemen are everywhere. >> reporter: while australian authorities charged four suspects for allegedly plotting a christmas day attack. >> it is a time of happiness, and joy. these terrorists sought to disrupt it. they have been thwarted. they are in custody. >> reporter: here at home, it's as much about prevention as it is response. in boston and philadelphia, more police have been assigned to patrol holiday sites since the berlin attack. while the fbi currently investigates claims that isis